string: add recoding from UTF-16 to UTF-32

I know there are system functions for this (and libiconv), but this is
so trivial and quick in our implementation.

UnitTests/compat.hpp

@@ -54,6 +54,17 @@ namespace Assert
 			throw std::runtime_error("not equal");
 	}
 
+	inline void AreEqual(const char32_t* a, const char32_t* b)
+	{
+#ifdef _WIN32
+		if (stdex::strcmp(a, b) != 0)
+			throw std::runtime_error("not equal");
+#else
+		if (wcscmp(reinterpret_cast<const wchar_t*>(a), reinterpret_cast<const wchar_t*>(b)) != 0)
+			throw std::runtime_error("not equal");
+#endif
+	}
+
 	template <class T>
 	void AreNotEqual(const T& a, const T& b)
 	{

UnitTests/main.cpp

@@ -26,6 +26,7 @@ int main(int, const char *[])
 		UnitTests::stream::file_stat();
 		UnitTests::stream::open_close();
 		UnitTests::stream::replicator();
+		UnitTests::string::strncpy();
 		UnitTests::string::sprintf();
 		UnitTests::unicode::charset_encoder();
 		UnitTests::unicode::normalize();

UnitTests/pch.hpp

@@ -107,6 +107,7 @@ namespace UnitTests
 	TEST_CLASS(string)
 	{
 	public:
+		TEST_METHOD(strncpy);
 		TEST_METHOD(sprintf);
 	};
 

UnitTests/string.cpp

@@ -12,6 +12,13 @@ using namespace Microsoft::VisualStudio::CppUnitTestFramework;
 
 namespace UnitTests
 {
+	void string::strncpy()
+	{
+		stdex::utf32_t tmp[0x100];
+		stdex::strncpy(tmp, u"This is a 🐔Test🐮.");
+		Assert::AreEqual(reinterpret_cast<const stdex::utf32_t*>(U"This is a 🐔Test🐮."), tmp);
+	}
+
 	void string::sprintf()
 	{
 		stdex::locale locale(stdex::create_locale(LC_ALL, "en_US.UTF-8"));